With the development of technology, mobile phones, digital cameras, tablet PCs, notebook computers and other portable electronic devices become more and more popular. The portable electronic devices are intended for personal use, with certain privacy. The information stored in the portable device such as phonebook, photos, personal information, etc., are privately owned. If the electronic device is lost, these data may be accessed by other people, causing unnecessary loss. Even though there are several ways using password to protect the electronic device from being used by the other people, the password may be easily to be spread or cracked, which lowers the security. Moreover, the user needs to remember the password so as to use electronic devices. If the user forgets the password, troubles would be brought to the user. Hence, personal fingerprint identification is utilized to achieve the purpose of personal identification, for enhancing the data security.
In general, the fingerprint identification system uses a capacitance sensing circuit to receive a touch of a finger. The capacitance sensing circuit is configured to receive the touch of the finger and forms a touch capacitance with the finger. The capacitance sensing circuit may convert the capacitance of the touch capacitance as an analog voltage signal. The analog voltage signal should be converted into digital voltage signals, and sent to the fingerprint identification module in the backend to determine whether the capacitance sensing circuit corresponds to a finger ridge or a finger valley of the finger. In other words, the analog voltage signal should be delivered to an analog-to-digital convertor (ADC) to be converted into digital signals. However, the ADC has high circuit complexity, occupies large circuit area, raises production cost, and consumes more power.